The present invention relates generally to High Electron Mobility Transistor (HEMT) devices and more particularly, to a HEMT device having a partially relaxed channel.
Referring to FIG. 1, a typical prior art HEMT device 10 comprises a buffer layer 12 which is grown on a substrate 14. A pseudomorphic channel 16 is grown on the buffer layer 12, and, a barrier layer 18 is grown on the channel 16. The barrier layer 18 typically includes a doping layer 20 disposed near the channel 16. A cap layer 22 is disposed on the barrier layer 18. The buffer layer 12 and the barrier layer 18 provide confinement of the carriers in the channel 16. The buffer layer 12 additionally isolates the channel 16 from the substrate 14. To complete the HEMT device 10, a gate 23 is positioned on the barrier layer 18 through a recess 24 formed in the cap layer 22. A source 24 and a drain 24 are positioned on the cap layer 22 on either side of the gate 23.
A thick channel 16 is desirable because a thicker channel 16 provides improved containment of the carriers which improves the electrical properties of the HEMT device 10. Therefore, it is desirable to fabricate the channel 16 as thick as possible. However, the thickness of the channel 16 is limited to a thickness which maintains the material in the channel 16 in a pseudomorphic state. In a pseudomorphic state, the material of the channel 16 is under strain with no defects in the material resulting from strain relaxation. Defects in the material of the channel 16 caused by strain relaxation begin to appear once the channel 16 is fabricated to a thickness greater than the critical thickness. When defects are present in the channel 16, the channel 16 is referred to as being either partially-relaxed or fully-relaxed. Partially-relaxed and fully-relaxed are terms used in the industry to distinguish between levels of defects in the material. A partially-relaxed material has some level of defects but not enough to cause the material to lose all beneficial material properties, whereas a fully-relaxed material has enough defects to cause the material to lose most of its beneficial properties.
Any level of defects in the material of the channel 16 are considered by those skilled in the art to be undesirable because defects are thought to reduce the DC and RF performance of the HEMT device 10. Therefore, the channel 16 of the prior art HEMT device 10 is only grown below the critical thickness, so that defects in the channel 16 can be avoided. This means that the typical HEMT device 10 has a relatively thin channel 16. A thin channel 16 poorly confines the carriers in the channel 16 which limits the gain and frequency response of the HEMT device 10.
What is desired therefore is a HEMT device which provides a higher gain and frequency response than provided by the prior art HEMT device 10.
The proceeding and other shortcomings of the prior art are addressed and overcome by the present invention which provides a HEMT device comprising a buffer layer disposed on a substrate. A partially-relaxed channel is disposed on the buffer layer and a barrier layer is disposed on the channel. A cap layer is disposed on the barrier layer and a gate is positioned on the barrier layer. A source and a drain are positioned on the barrier layer on opposite sides of the gate.
In a second aspect the channel is fabricated of a material which, when disposed to a first thickness is pseudomorphic; and, when disposed to a second thickness, is fully-relaxed. The channel is fabricated to a thickness intermediate the first and second thicknesses.
In a third aspect, the present invention provides a method for fabricating a HEMT device having a gate, a source and a drain. A buffer layer is deposited on a substrate. A partially-relaxed channel is deposited on the channel and a barrier layer is deposited on the channel. A cap layer is deposited on the barrier layer and the gate is positioned on the cap layer through a recess formed in the cap layer. The source and the drain are positioned on the cap layer on opposite sides of the gate.